RF transmitters that attempt to provide linear amplification may suffer from a variety of signal distortions. In such applications, real-world RF amplifiers fail to provide perfectly linear amplification, causing spectral regrowth to occur. Since modern regulations place strict limitations on the amount of spectral regrowth that may be tolerated, any signal distortion resulting from nonlinear amplification poses a serious problem for RF transmitter designs. In addition, any linear distortion in the transmitted RF communication signal is undesirable because linear distortion must be overcome in a receiver, often by necessitating transmission at greater power levels than would otherwise be required. Linear distortions also complicate the spectral regrowth problem.
A variety of well known RF power amplifier and other analog component design techniques may be employed to ensure that nonlinear amplification and other forms of distortion are held to a minimum. But as such techniques get more exotic, the analog component costs increase, and often increase dramatically. Accordingly, predistortion may be a desirable alternative to the use of exotic and expensive analog components, such as highly linearized RF power amplifiers.
Digital predistortion has been applied to digital communication signals prior to signal processing in analog components to permit the use of less expensive power amplifiers and also to improve the performance of more expensive power amplifiers. Digital predistortion refers to digital processing applied to a communication signal while it is still in its digital form, prior to analog conversion. The digital processing attempts to distort the digital communications signal in precisely the right way so that after inaccuracies are applied by linear amplification and other analog processing, the resulting transmitted RF communications signal exhibits negligible residual distortion. To the extent that amplifier nonlinearity is corrected through digital predistortion, lower-power, less-expensive amplifiers may be used, the amplifiers may be operated at their more-efficient, lower-backoff operating ranges, and spectral regrowth is reduced. And, since the digital predistortion is performed through digital processing, it should be able to implement whatever distortion functions it is instructed to implement in an extremely precise manner and at reasonable cost.
The more effective predistortion techniques obtain knowledge of the way in which analog components distort the communications signal in order to craft the proper predistortion-transfer functions that will compensate for distortion introduced by the analog components. A predistortion technique that is disclosed in the above-listed Related Inventions section hereof uses a collection of adaptive equalizers to determine, implement, and continuously or repeatedly revise such predistortion-transfer functions. One adaptive equalizer filters a baseband communication signal, while other adaptive equalizers filter “basis functions” that are functionally related to the baseband communication signal raised to various powers. Each of the predistortion adaptive equalizers has tap coefficients that define how to predistort the baseband communication signal or basis functions. The tap coefficients are adjusted in response to a feedback signal which provides knowledge about the way in which the analog components are distorting the communication signal at each instant. As a result, feedback loops are formed and tap coefficients are continuously or repeatedly adjusted so that spectral regrowth and linear distortion are minimized.
This prior technique taught that tap coefficients are adjusted through the use of a type of Least Mean Square (LMS) algorithm. For the linear adaptive equalizer that filters the baseband communication signal, the LMS algorithm generally identified correlation between the baseband communication signal and an error signal formed in response to the feedback signal. The correlation function was implemented by a long sequence of mathematical multiplications. The sequence of multiplications was integrated, and the integration results were used in forming tap coefficients. The integration operations caused tap coefficients to adjust until steady-state integrator values were reached where distortions in the in-band portion of the communication signal were minimized, and then to track any changes.
But the feedback signal also includes components other than a distorted version of the original baseband communication signal. To the extent that the feedback signal includes noise or other components that do not correlate with the baseband communication signal, the tap coefficients are not affected. But some of the other components are intermodulation products, albeit at low residual levels due to the operation of nonlinear predistortion. And, a portion of the intermodulation products may be slightly correlated to the baseband communication signal because this signal is the source of some of the intermodulation products as it is upconverted and passed through a nonlinear device, such as an imperfectly linear power amplifier. Correlation between the baseband communication signal and such intermodulation products cause tap coefficients to be less accurate then they could be, although any inaccuracy is usually at a very low level.
On some occasions, however, a regenerative feedback loop can potentially form. Generally, tap coefficients in the adaptive equalizer that serves as the linear predistorter may adapt in a direction that can reduce linear distortion, but in so adapting also worsen those correlated intermodulation products that are present at a low level in the feedback signal. When this happens, the adaptive equalizer may be insufficiently effective at reducing linear distortion because of the interference caused by correlated intermodulation products in the tap adjustment algorithm. And, in rare situations, adapting tap coefficients to address linear distortion might possibly worsen those correlated intermodulation products that are present at a low level in the feedback signal to a greater extent than the linear distortion is reduced. In this scenario, an unstable feedback loop would result, causing undesirable distortions to appear in the transmitted RF communication signal.